5 The electric generator

• What if you turn the shaft of a motor by hand, instead of supplying it with current?
• The process reverses and the motor produces electrical energy.
• This is the principle of the generator.

Activity.

• Connect a small DC motor to an LED (or small bulb), fix a small fan on its shaft and turn the fan by hand.
• The lamp lights; turning faster makes it brighter.
• (An LED lights only when current passes in one direction; if it does not light, reverse its connections.)

An electric generator converts mechanical energy into electrical energy.

Working principle.

• A coil is rotated by some source of mechanical energy between the poles of a magnet.
• As the coil cuts the magnetic field lines, an electromotive force (emf) is generated in it, driving a current through the external circuit.
• The emf generated increases when the coil's rotational speed increases (and also with a stronger magnet or more turns).

Life application: bicycle dynamo.

• A small generator pressed against the wheel is turned by friction as the wheel rotates; it gives electricity to the lamps without batteries.
• The faster the wheel turns, the larger the emf and the brighter the lamp.

Uses:

1. Power stations. Turbines driven by water falls, steam or wind turn coils in large generators to power cities and factories. Power stations use generators producing alternating current (AC), the type used in homes. (A generator with a split-ring commutator produces direct current.) Advantages of AC: it can be transmitted over long distances with small energy loss, and it can be converted to DC when needed.
2. Emergency generators in homes, hospitals and factories for power cuts.